1. Field of the Invention
The present invention relates generally to gyromagnetic compass systems for navigable craft and more specifically to an analog, all solid state gyromagnetic compass system.
2. Description of the Prior Art
Systems in which the output of a gyroscope is slaved to the output of a flux valve have been known to the art for many years and have indeed become the standard heading reference for modern commerical and military aircraft. The basic form of such a system is shown in U.S. Pat. Nos. 2,357,319; 2,383,416; and 2,969,208, all assigned to the assignee of the present invention. In these patents, the directional reference provided by the gyroscope is electromechanically slaved to the direction of the earth's magnetic field as detected by a flux valve, by comparing the direction of the earth's field with the gyro reference direction and applying the difference therebetween to a torquer on the gyro thereby causing the gyro to precess such that the error reduces to zero and the gyro reference corresponds to magnetic north.
The flux valve senses the horizontal component of the earth's magnetic field by maintaining the sensing elements of the valve in a generally horizontal plane through a pendulous mounting thereof in the aircraft. Therefore, if the craft is subjected to acceleration forces, the flux valve information may be in error as a result of its coupling with the vertical component of the magnetic field. However, in straight and level unaccelerated flight, the flux valve provides stable directional information. On the other hand, the gyroscope provides stable information during turns and short term accelerations of the craft but is subject to a long term drift usually inherent in gyroscopic devices. Since in a conventional gyromagnetic compass system the flux valve-gyro error is applied to a torquer on the gyro to precess the same at a relatively slow rate, i.e., about two degrees or three degrees per minute, the gyro serving to integrate the flux valve information to thereby remove the undesired short term errors. However, short term maneuvers of the craft will immediately be detected by the gyro but will not be seen by the slaving loop. Long term gyro drift on the other hand will not be allowed to build up on long term basis because the gyro is slaved to the flux valve.
Many variations of the basic gyromagnetic compass configuration just described have been proposed and successfully implemented. One of these is an arrangement wherein the gyro spin or reference axis is not slaved to the magnetic meridian and the integration of the flux valve information is accomplished by an electromechanical integrator, the gyro information providing only the short term data stabilization supplied through a fast response electromechanical follow-up loop. Such a system is described in Applicant's assignee's U.S. Pat. No. 2,699,612.
Further improvements in the latter type of configuration are illustrated in Applicant's assignee's U.S. Pat. No. 3,691,643 wherein long term data from the flux valve and short term gyro data are combined, for example, at a differential synchro on the gyro, to produce an error signal or synchronizing angle signal for electronically "slaving" the gyro. The error is applied to the differential signal in such a manner as to reduce the error on a long term basis while permitting short term gyro data to be directly transmitted to the system output. This system is an all solid state system but employs digital computation technique which while accurate and reliable are relatively expensive in terms of hardware and software mechanization.